The present invention relates to improvements in the art of packaging foodstuffs, especially cook-in foods such as for example ham, beef, and turkey breasts.
Manufacturers and wholesalers use flexible thermoplastic packaging films to provide economical, sanitary containers, which help protect and/or preserve the freshness and wholesomeness of their products. These films are often sold in bag form. For example, a single or multilayer film is made into bags using a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags. These films and bags may be printed and may also be uniaxially or biaxially oriented, heat shrinkable, irradiated, or may contain film layers which are abuse resistant or puncture resistant or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrough. Frequently, multilayer films having one or more barrier layers to oxygen and/or moisture such as: Saran (a polyvinylidene chloride copolymer); a modified saran e.g. MA-Saran containing methyl acrylate polymer units; ethylene vinyl alcohol copolymer; nylon; or acrylonitrile may be used with a heat sealing layer such as a copolymer of ethylene and vinyl acetate (EVA) to produce bags for packaging oxygen and/or moisture sensitive foods e.g. processed pork or fresh red meat. Such bags help preserve meat in its original condition by preventing or reducing moisture loss and chemical changes in the meat structure due to oxidation reactions.
A typical packaging bag has three sides heat sealed by the bag manufacturer leaving one open side to allow product insertion. For example, a processor may insert fresh, frozen or processed meat, ham, poultry, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other products making a final seal to hermetically enclose the product in the bag. This final seal may follow gas evacuation (i.e. vacuum removal) or replacement of the gaseous environment within the bag by one or more gases to provide some advantage such as to assist product preservation. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer although the actual heat sealing equipment may vary.
Thus, bags are made: by transversely sealing tubular stock of monolayer or multilayer film and cutting off the tube portion containing the sealed end; by making two spaced apart transverse seals on tubular stock and cutting open the side of the tube; by superimposing flat sheets of film and sealing on three sides; or by folding a flat sheet and sealing two sides.
Generally, heat seals are made by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the plastic film layers.
A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars of which at least one is heated to cause the layers to fusion bond by application of heat and pressure across the area to be sealed. For example, bags may be made from a tube stock by making one hot bar bottom seal transverse to a tubular film. Once the bottom seal is made, the tube stock is transversely cut to form the mouth of the bag.
After a product is inserted, the bag is typically evacuated and the bag mouth sealed to enclose the product. At one time, the standard method for sealing was to fasten a clip around the mouth of the bag. However, heat sealing techniques are now also commonly employed to produce the final closure of the bag. For example, a bag mouth may be either hot bar sealed or impulse sealed. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name “impulse”) to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are typically cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
Relative to hot bar seals, impulse seals may be made faster because of the quick cool down of the ribbon following the heat impulse. Impulse seals are also generally narrower giving an improved package appearance, but narrower seals also leave less margin for error in the production of continuous sealed edges. Less area is usually bonded in an impulse seal relative to a hot bar seal, thus the performance of the film's sealing layer is more critical.
Disadvantageously, the film in the seal area often becomes extruded during impulse sealing of known films. This results in thinning of the film and a reduction of film strength in the seal area. In extreme situations, the thinned film is severed or pulled apart. Those skilled in the art refer to severely extruded seals to as “burn through” seals. A “burn through” seal does not have adequate strength or integrity to protect the packaged product. One attempt to solve this “burn through” problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a film made from cross-linkable polymer resins causes resin layers in the film to crosslink. Under controlled conditions, crosslinking by irradiation raises and may also broaden the temperature range for heat sealing, and depending upon the film composition may also enhance puncture resistance of the film. If the heat sealing layer of the thermoplastic film is crosslinked too heavily, it is more difficult to fusion bond which makes achieving strong seals difficult, particularly by impulse sealing. All bag seals must maintain their integrity to preserve and protect enclosed products, especially food products.
There must be a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid or solid materials between the bag exterior and interior. This is particularly, necessary when the package is made of heat shrinkable film and is to be immersed in hot water to shrink the film against the packaged article since such shrinkage increases the stress on these seals. It is even more critical where the packages are to be immersed at sufficient times and temperatures for pasteurization or cooking. Thus, there is a continuing need for films which can be made into bags having strong seals especially those formed by hot bar and/or impulse sealing. Such films should provide strong seals able to withstand a range of temperatures and also be able to make such seals over a wide sealing temperature range without burn through.
Variations in sealing temperatures, times and pressure are known to exist from one brand or type of sealer to another and also between different sealing machines sold under the same brand. This increases the desirability for films that may be usefully sealed on different sealing machines and over a wide range of temperatures to produce strong integral seals.
Another heat sealing problem is that of inadvertent folding. Normally, a heat seal is made by applying heat and pressure across two portions of film. But occasionally, the area to be sealed will be inadvertently folded to produce a section of film having four or six film portions that are pressed between the opposing sealer bars. In such situations, it is desirable to be able to seal the film without burn through. A wider impulse heat sealing temperature range is indicative of a greater latitude in sealing through folds than a narrower range.
Another problem during heat sealing is that of excessively high tear propagation strengths. Lower tear propagation strengths are an advantage in heat sealing operations using impulse sealing technology where the sealing apparatus both seals and cuts the film with the film trim being removed by tearing along the cut. Low tear propagation strengths enable quick trim removal without damage to the seal, film or bag.
A very demanding application for heat shrinkable, heat sealable thermoplastic flexible films is for processing meats. Bacterial contamination during food processing e.g. by Listeria monocytogenes is of great concern. To address health and safety concerns with processed foods, some processors have adopted a surface heat treatment at elevated temperatures sufficient to kill bacteria on already cooked food.
In some demanding applications, a food product such as a ham is sealed inside a plastic processing bag or film in which the ham is cooked, refrigerated, shipped and subsequently displayed for retail sale.
In a more common demanding application, food such as a turkey breast, ham, or beef is cooked in a pan, net, or processing film from which the cooked food is removed for further processing such as: slicing; smoking in a smokehouse; treatment with colorants and/or flavorants such as caramel, spices, liquid smoke or honey; glazing; and/or removal of liquid (known as purge) resulting from e.g. the cooking process. Following this further processing, the food product is packaged, often in a printed bag, for shipment and sale. The cooked food is typically placed into a heat sealable, heat shrinkable bag which is then emptied of atmospheric gases by vacuum, heat sealed and subjected to a film shrinking operation usually in a water tank at elevated temperature for a brief period of time to produce a compact attractive package. During these steps that follow cooking and occur prior to packaging for shipment and sale, the food product surface is subject to environmental contamination, for example, by airborne particles, microbes, and, dust. The risk from contamination after packaging may be minimized by surface pasteurizing the encased sealed package e.g. in a water bath or steam chamber held at elevated temperatures for a time sufficient to provide the desired degree of protection from microbial contamination and growth. The time and temperatures of this post-cooking pasteurization step may vary widely.
Significantly this surface treatment is in addition to the cooking or pasteurization process and follows hermetically sealing the cooked or pasteurized food in a plastic packaging film. In this demanding use, this “post-cooking pasteurization” surface treatment is performed after placing the food into the packaging film that will remain on the pasteurized product through sale to an ultimate customer. Often the films are printed with consumer information and brand identification and frequently at least a portion of the film is clear to allow viewing of the enclosed product. Therefore, optical properties and film appearance are important for consumer appeal and sale.
This “post-pasteurization” film must perform a variety of functions well. It must be puncture resistant and have strong seals at the elevated temperatures encountered in the shrinking operation, and also with the post-cooking pasteurization process. It should also keep tight conformation of the film around the product at refrigeration temperatures with an attractive appearance and act as a good barrier to oxygen, moisture and environmental contaminates.
Past attempts at providing improved puncture resistance and heat sealing in films, while making some progress, still leave room for improvement. Variability in heat sealing equipment and process parameters continue to produce bags with weak seals which are subject to tearing and stress on the seals during cutting operations, which are subject to burn through, which fail to seal through folds, and which produce leaking bags having discontinuous seals and which are not sufficiently resistant to punctures. It would be highly desirable to have biaxially stretched, heat shrinkable films and bags which are highly puncture resistant and/or whose heat sealing layer in particular and film construction in general allows greater flexibility and variability in heat sealing process parameters while producing strong, integral, continuous seals rapidly and with a lower failure rate relative to prior art films and bags.
Accordingly, one object of the present invention is to provide a novel film structure having an improved combination of properties.